1. The Field of the Invention
The present invention relates to the purification of ammonia. More particularly, the present invention provides methods, materials and apparatus for removing oxygen from ammonia. More specifically, the present invention provides a novel getter material that removes oxygen from ammonia at low temperatures.
2. Background
Pure gases are essential in a number of fabrication processes important in the semiconductor manufacturing industry. Gas purity is positively correlated with the yield of integrated circuits in semiconductor manufacturing and is especially critical in the fabrication of advanced modern transistors which are easily damaged by small amounts of contaminants because of their small feature widths.
Ammonia (NH3) is a process gas of primary importance in the semiconductor industry for the formation of nitride layers in electronic transistors through chemical vapor deposition and epitaxy processes. More specifically, ammonia is most commonly used for the formation of silicon nitride and silicon oxynitride films by direct nitridation of silicon oxide. Silicon nitride is both a more effective barrier to alkali ion migration and more resistant to thermal oxidation than silicon oxide and is thus particularly useful as a cover layer in metal-on-silicon (MOS) technology and as a mask when selective oxidation of the semiconductor is required. Silicon oxynitride films have physical properties intermediate between silicon and silicon nitride and have enormous potential for use in ultra large scale integrated circuits and as a passivating layer in gallium arsenide materials. Growing films of silicon nitride and silicon oxynitride requires ammonia of very high purity. Oxygen is a particularly harmful contaminant because its high chemical reactivity leads to its ready incorporation as an impurity into films during thermal nitridation of silicon oxide.
Getter materials comprise metals and metal alloys that sorb various gas molecules such as carbon oxides (COx, where x is 1 or 2), hydrogen (H2), nitrogen (N2), oxygen (O2) and water (H2O) and thus have been widely used for the purification of gases. In this application, advantage is taken of the ability of getter materials to sorb certain various compounds preferentially from gaseous mixtures. For example, such uses include the removal of carbon oxides (CO and CO2), nitrogen (N2), methane (CH4), and oxygen (O2) from hydrogen. The gettering function, and even the particular species that are sorbed, depends on the temperature of the getter materials.
Particularly useful are non-evaporable getter (“NEG”) materials, which include zirconium- or titanium-based alloys in combination with elements such as aluminum, vanadium, iron, nickel or other transition elements or their combinations. Examples of getter materials include the alloy having the composition Zr 84%—Al 16% by weight, which is manufactured and sold by SAES Getters S.p.A. (Milan, Italy) under the name “St 101®”, and the alloy having the composition Zr 70%—V 24.6%—Fe 5.4% by weight, also manufactured and sold by the SAES Getters under the tradename “St 707”.
The use of getter materials to remove oxygen from ammonia has been disclosed, for example, in European Patent No. 484-301-B1 assigned to the SAES Getters and incorporated herein by reference for all purposes, which describes contacting oxygen containing ammonia with the getter alloy St 707 that selectively absorbed oxygen from the gaseous mixture. However, despite the effectiveness of the patented process in purifying ammonia, a number of practical difficulties related to the 100–150° C. operating temperature of the getter alloy prevent significant industrial use of this method. First, St 707 decomposes ammonia at these elevated temperatures. Second, if the ammonia is contaminated with large concentrations of oxygen, then oxygen absorption by St 707, which is extremely rapid at the operating temperature of the getter alloy, can lead to an exothermic autocatalytic reaction that can explosively destroy the gas purification device. Finally, gas purification processes that operate at ambient temperature (i.e., at or about room temperature or 25° C.) are preferred in the semiconductor industry because of cost considerations and engineering simplicity.
Therefore, a process that would remove oxygen from ammonia at about room temperature is highly desirable.